In voice communication systems, tones are frequently used for signaling. A dual tone, commonly referred to simply as a "tone", is represented by two true tones whose frequencies are separated in bandwidth, in order to avoid false tone detections. Thus, if normal speech patterns or noise produce a signal with a high amount of energy near one true tone frequency, it is unlikely that there will be enough energy near the other true tone frequency to result in a spurious tone detection. This type of tone signaling is utilized, for example, in dual tone multi-frequency (DTMF) signaling used in North American telephone systems.
Known tone detectors typically employ the band separation technique, and these tone detectors usually fall within one of two categories. One type, known as a band reject tone detector, filters an input signal through two band select filters corresponding to the two DTMF frequency bands. A band select filter associated with the low DTMF frequency band, rejects signal frequencies in the high DTMF frequency band, and vice versa. The outputs of the band select filters are processed through signal limiters and then passed through resonators. Resonators tuned to each of the DTMF frequencies provide an output signal of sufficient energy to trip a subsequent comparator when the input signal has a sufficient magnitude at one of the DTMF frequencies. If the outputs of resonators in both the low and high DTMF frequency bands trip their respective comparators, then the tone detector provides a tone detect output signal to indicate the occurrence of a DTMF tone.
However, the band reject tone detector is susceptible to missing valid tones. If there is enough energy in other signals while a tone is present, the speech interferes with the tone and the band reject tone detector is unable to discern the tone. For example, speech interference may be due to a recorded announcement, customer speech, an operator, or background noise.
A second type of tone detector is known as a bandpass tone detector. In the bandpass tone detector, the input signal is first passed through bandpass filters corresponding to the low and high tone ranges. If a signal within the passband of the filter has a sufficient amplitude, it will trip a comparator. If two true tones are detected, the bandpass tone detector will provide a tone detect indication. While immune from missing tones, the bandpass tone detector is nonetheless subject to false tone detects, such as when random speech has a large amount of energy in two tone frequencies.
Some band separation tone detectors attempt to avoid the problem of speech interference by muting a local line before sending a tone, and only detecting tones during silent periods. However, some tone signaling systems allow tones at all times and the system cannot be continuously muted. Thus, a tone detector which is able to correctly discern tones in the presence of speech is desirable.